An encoded watermark is one type of forensic aid that has been proposed for identifying the source of an illegal copy of a motion picture. When provided with a suitable watermark, a projected motion picture image carries within itself identifying data that persists after any type of copying operation, whether an unauthorized copy was made in the theater using a portable digital camera or duplicated from illegally obtained film prints.
A watermark scheme for motion picture images must meet the same base set of requirements that apply to watermark approaches for protection of still images and many other types of documents. Key requirements for effective motion picture watermark implementation include the following:                (a) Not detectable to the viewer. A visible watermark would be annoying and would detract from image quality and from the overall film viewing experience.        (b) Robust. At the same time, the watermark must be durable so that it can be identified with suitable detection techniques and cannot be easily filtered out. Ideally, some tolerance should be allowed for shifting of the watermark within an image frame, to allow straightforward detection with conventional scanning equipment.        (c) Workable with existing emulsion formulations. For ease of acceptance and implementation, a motion picture watermarking scheme should not require changes to emulsion formulations or processing treatments currently in use. This would allow a watermark application to be introduced in a phased manner, over a period of time or with selected films, without disruption of film manufacture or processing operations.        
Also of interest would be a watermark implementation that provides the following advantages:                (d) Speed. Driven by economic factors and high volumes, film fabrication and subsequent processing are performed at high speeds. There would be considerable reluctance among film manufacturers or processors to adapting a watermarking method that slowed motion film production or processing.        (e) Ease of integration. In order to make watermarking feasible, any method that is adopted must allow for ease of implementation within existing film production and processing workflows. Pre-exposure of a photosensitive film medium would allow watermark application at the source of manufacture of distributed print films, allowing film distributors the option to use or to ignore watermarking.Added to these requirements are overall concerns for minimizing cost and maintaining overall quality of the projected image.        
A number of approaches for image watermarking have been proposed in the art. For example:                U.S. Pat. No. 5,734,752 (Knox) discloses a method for digital image watermarking of a document using a randomized, stochastic screen pattern;        U.S. Pat. No. 5,315,098 (Tow) discloses the use of glyphs, small lines of a few pixels length, within an image, the glyphs oriented at various angles to store watermarking data; Commonly-assigned U.S. Pat. Nos. 5,752,152 and 5,864,742 (Gasper et al.) disclose the use of pre-exposed microdots distributed on a photosensitive imaging medium used for images that are copyright-protected;        U.S. Pat. No. 6,166,750 (Negishi) discloses a watermarking scheme by adding pixel data to a copied image;        U.S. Pat. No. 6,370,258 (Uchida) discloses a number of watermarking approaches, such as varying pixel dot position, modulating density of a color component, or adding high-frequency image data;        U.S. Pat. Nos. 5,457,540 (Kajita) and U.S. Pat. No. 5,557,416 (Sasanuma et al.) both disclose watermark formation by changing dot density of an applied color in an image;        U.S. Pat. No. 6,256,110 (Yoshitani) discloses using a plurality of inks of nearly the same color for encoding data in an image;        U.S. patent application Ser. No. US 2003/0012569 (Lowe et al.) discloses a method for pre-exposing a photosensitive imaging medium to apply a watermarking (steganographic) pattern, using a custom-designed, movable barrel-shaped imaging member that is rolled along the surface of the unexposed film or using illumination gated by a narrow slit; and        U.S. Pat. No. 6,407,767 (Klees et al.) discloses an apparatus for exposing sensitometric (calibration) patches and barcode data onto color negative film during the manufacturing process. Two white light flashlamps, one large and one small, are used to expose gray scale patches onto the processed film. While the disclosed apparatus can provide accurate color-neutral exposure onto stationary film as part of a manufacturing test process, its power is not sufficient for exposing film that is traveling at speeds in excess of a few feet/second. Moreover, resolution provided by this technique is not sufficient for minimizing the visible effects of the embedded data.        
These and similar approaches have been applied, with varying degrees of success, to the watermarking of documents and still images. However, none of the approaches outlined in these disclosures is well suited for motion picture film watermarking, in light of the requirements listed in (a) through (e) above. Methods that apply added color to an image could be used, in the spirit of the above-noted U.S. Pat. Nos. 5,457,540; 6,166,750; and 5,557,416; however, this operation must be provided by the motion picture film printing lab and distributor. As is noted above, changes to the existing, high-volume workflow are not likely to gain acceptance. High-cost printing apparatus, such as would be needed using the approach of the above-noted U.S. patent application Ser. No. 2003/0012569, would not be desirable, nor would color or emulsion formulation changes, such as might be suggested in the approach of U.S. Pat. No. 6,256,110 noted above. Methods that form a two-dimensional watermark, such as those disclosed in the above-noted U.S. Pat. Nos. 5,752,152; 5,864,742; 5,734,752; 6,370,258; and 5,315,098, would require pre-exposure of an area of the film. With film moving through fabrication stages at high speeds, extremely high intensity and very short exposure times are required. Reciprocity failure problems in film exposure, heightened by these requirements, further complicate the problem of applying a two-dimensional watermark.
Thus, it can be seen that the prior art solutions noted above are not well suited to the requirement for pre-exposure of a watermark following high-speed film coating and drying and preceding high-volume packaging operations. Further, considering the needed response and refresh times, most types of image-forming devices simply operate too slowly for high-speed watermarking applications. There is, therefore, a need for an apparatus and method for watermark pre-exposure onto a photosensitive film medium during its manufacture that meets demanding requirements for high-speed and low cost.